Effect of grinding and heat treatment on the mechanical behavior of zirconia ceramic

Gabriela Freitas RAMOS Gabriel Kalil Rocha PEREIRA Marina AMARAL Luiz Felipe VALANDRO Marco Antonio BOTTINO About the authors

Abstract

The present study investigated the effect of grinding on roughness, flexural strength, and reliability of a zirconia ceramic before and after heat treatment. Seven groups were tested (n = 15): a control group (labeled CG, untreated), and six groups of samples ground with diamond discs, simulating diamond burs, with grits of 200 µm (G80); 160 µm (G120), and 25 µm (G600), either untreated or heat-treated at 1200°C for 2 h (labeled A). Yttria tetragonal zirconia polycrystal discs were manufactured, ground, and submitted to roughness and crystalline phase analyses before the biaxial flexural strength test. There was no correlation between roughness (Ra and Rz) and flexural strength. The reliability of the materials was not affected by grinding or heat treatment, but the characteristic strength was higher after abrasion with diamond discs, irrespective of grit size. The X-ray diffraction data showed that grinding leads to a higher monoclinic (m) phase content, whereas heat treatment produces reverse transformation, leading to a fraction of m-phase in ground samples similar to that observed in the control group. However, after heat treatment, only the G80A samples presented strength similar to that of the control group, while the other groups showed higher strength values. When zirconia pieces must be adjusted for clinical use, a smoother surface can be obtained by employing finer-grit diamond burs. Moreover, when the amount of monoclinic phase is related to the degradation of zirconia, the laboratory heat treatment of ground pieces is indicated for the reverse transformation of zirconia crystals.

Ceramics; Dental Alloys; Dental Materials

Introduction

Advances in CAD/CAM systems have enabled the optimization of ceramic materials, extending their clinical applications. Yttria tetragonal zirconia polycrystal (Y-TZP) stands out among other restorative dental materials due to its high chemical stability and biocompatibility, and superior mechanical properties.11. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials. 1999;20(1):1-25. doi:10.1016/S0142-9612(98)00010-6,22. Kosmač T, Kosmač T, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res. 2000;53(4):304-13. doi:10.1002/1097-4636(2000)53:4<304::AID-JBM4>3.0.CO;2-S These highly favorable mechanical properties allow for the application of all-ceramic Y-TZP crowns and bridges in the posterior region, also reducing the amount of tooth structure removal during preparation, since a smaller thickness of the restorative material is required. These characteristics are very attractive for applications in dental prostheses, in which strength and esthetics requirements are crucial33. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater. 2008;24(3):299-307. doi:10.1016/j.dental.2007.05.007and/or there is a narrower interocclusal space.

Zirconia is a polymorphic material existing in three different crystalline forms, stabilized in tetragonal phase at room temperature. Yttrium oxide proved to be an excellent option in this context, since it creates a fine-grain microstructure (3Y-TZP).11. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials. 1999;20(1):1-25. doi:10.1016/S0142-9612(98)00010-6 Nevertheless, the tetragonal to monoclinic (t→m) phase transformation may be triggered by different stimuli, such as stress concentration or low-temperature aging, in the presence of humidity, leading to low-temperature degradation.44. Souza ROA, Valandro LF, Melo RM, Machado JPB, Bottino MA, Ozcan M. Air-particle abrasion on zirconia ceramic using different protocols: effects on biaxial flexural strength after cyclic loading, phase transformation and surface topography. J Mech Behav Biomed Mater. 2013;26:155-63. doi:10.1016/j.jmbbm.2013.04.018

Despite the advances in CAD/CAM manufacturing systems, and pre-sintered ceramic blocks, fine adjustments of already sintered zirconia surfaces are still performed by dental technicians or clinicians,55. Aboushelib MN, Feilzer AJ, Kleverlaan CJ. Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach. Dent Mater. 2009;25(3):383-91. doi:10.1016/j.dental.2008.09.001 in some cases, for a better fitting of a prosthetic restoration. These manual procedures could trigger the t→m phase transformation and grinding, creating a compressive stress layer, due to an increase in the volume of the material (zirconia) by approximately 3%, in addition to producing some surface defects in the material. When the depth of these defects is greater than the thickness of the compressive layer, the defects can act as stress concentration zones, which can impair the mechanical properties.66. Kosmač T, Kosmač T, Jevnikar P, Funduk N, Marion L. The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater. 1999 Nov;15(6):426-33. doi:10.1016/S0109-5641(99)00070-6,77. Guazzato M, Quach L, Albakry M, Swain MV. Influence of surface and heat treatments on the flexural strength of Y-TZP dental ceramic. J Dent. 2005;33(1):9-18. doi:10.1016/j.jdent.2004.07.001,88. Quinn GD, Ives LK, Jahanmir S. On the nature of machining cracks in ground ceramics: Part I: SRBSN strengths and fractographic analysis. Machin Sci Technol. 2005;9(2):169-210. doi:10.1081/MST-200059038 Nevertheless, when the depth of these defects is lower than the thickness of the compressive stress layer, crack propagation is hindered and catastrophic failures are avoided by the surrounding compressive stresses.99. Chevalier J, Gremillard L, Deville S. Low-temperature degradation of zirconia and implications for biomedical implants. Annu Rev Mater Res. 2007;37(1):1-32. doi:10.1146/annurev.matsci.37.052506.084250,1010. Kim JW, Covel NS, Guess PC, Rokow ED, Zhang Y. Concerns of hydrothermal degradation in CAD/CAM zirconia. J Dent Res. 2010;89(1):91-5. doi:10.1177/0022034509354193 However, grinding can contribute to premature aging, causing the material to lose its ability to prevent crack propagation.1111. Preis V, Schmalzbauer M, Bougeard D, Schneider-Feyrer S, Rosentritt M. Surface properties of monolithic zirconia after dental adjustment treatments and in vitro wear simulation. J Dent. 2015;43(1):133-9. doi:10.1016/j.jdent.2014.08.011

A strong correlation has been reported in the literature between the amount of monoclinic phase and degradation of the mechanical properties of zirconia.1212. Cattani-Lorente M, Scherrer SS, Ammann P, Jobin M, Wiskott HWA. Low temperature degradation of a Y-TZP dental ceramic. Acta Biomater. 2011;7(2):858-65. doi:10.1016/j.actbio.2010.09.020,1313. Zhang Y, Lawn BR, Malament KA, Thompson VP, Rekow ED. Damage accumulation and fatigue life of particle-abraded ceramics. Int J Prosthodont. 2006;19(5):442-8. Therefore, procedures that prevent t→m phase transformation or that cause reverse m→t transformation should be investigated as alternative ways to prevent the degradation of mechanical properties. Different heat treatment protocols have been proposed to induce surface healing by promoting reverse m→t transformation. These protocols relieve the stress present in the compression layer formed as well as the residual stresses induced on the zirconia surface.1414. Deville S, Chevalier J, Gremillard L. Influence of surface finish and residual stresses on the ageing sensitivity of biomedical grade zirconia. Biomaterials. 2006;27(10):2186-92. doi:10.1016/j.biomaterials.2005.11.021

The aim of this study was to assess the effect of diamond grinding of zirconia, with and without heat treatment, on strength and surface characteristics of this ceramic material. The tested hypotheses are that: (1) the grinding of zirconia with larger grit-size devices leads to a decrease in flexural strength and to an increase in monoclinic content and in surface roughness; and (2) heat treatment restores the original characteristics of the material.

Methodology

The materials used in this study are listed in Table 1.

Table 1
Material, respective manufacturers, city and country.

Lava Frame (3M ESPE) is a tetragonal zirconia polycrystal partially stabilized with 3mol-% yttria. This ceramic presents a very fine microstructure, containing fine grains and few slightly large grains (mean diameter of 104.9 nm ± 48.5).1515. Pittayachawan P, McDonald A, Petrie A, Knowles J. The biaxial flexural strength and fatigue property of LavaTM Y-TZP dental ceramic. Dent Mater. 2007;23(8):1018-29. doi:10.1016/j.dental.2006.09.003

Preparation of specimens

Disc-shaped specimens were manufactured according to ISO 6872/2008.1616. International Organization for Standards. ISO 6872: Dental ceramic. Geneve: International Organization for Standards; 1999. Pre-sintered blocks of Y-TZP (Lava Frame) were ground into cylinders with SiC paper (3M) under water cooling and sectioned with a precision saw (ISOMET 1000, Buehler, Lake Bluff, USA) into slices of 18 mm (Ø) × 1.6 mm (thickness). To remove irregularities left by the cutting tool, the disc surfaces were finely ground with a 1200-grit SiC paper, and the samples were then subjected to firing at 1530°C for 2 h in an especially designed furnace (Zyrcomat T, Vita Zahnfabrik, Bad Säckingen, Germany), according to the manufacturer’s instructions. The final disc dimensions were 14 mm × 1.2 mm. The discs were divided into seven groups: CG (control group)- discs without grit blasting and without heat treatment; G80- discs ground with a 200 µm grit paper and without heat treatment; G80A- discs ground with a 200 µm grit paper and heat-treated; G120- discs ground with a 160 µm grit paper and without heat treatment; G120A- discs ground with a 160 µm grit paper and heat-treated; G600- discs ground with a 25 µm grit paper and without heat treatment; and G600A- discs ground with a 25 µm grit paper and heat-treated. Both surface and heat treatments are explained in what follows.

Surface treatment

The samples were attached to a polishing machine (AutoMet 250, Buehler, Lake Bluff, USA ) and subjected to abrasion with diamond discs (Dia-Grid Diamond Discs, Allied High Tech Products) for 60 s using a 60 N load, at 300 rpm on the turntable (clockwise) and at 40 rpm on the table top device (counter-clockwise), under constant water cooling, with one of the following grit sizes: 80 (200 µm), 120 (160 µm) or 600 (25 µm). These grit sizes were selected according to Kim et al.,1010. Kim JW, Covel NS, Guess PC, Rokow ED, Zhang Y. Concerns of hydrothermal degradation in CAD/CAM zirconia. J Dent Res. 2010;89(1):91-5. doi:10.1177/0022034509354193 to simulate super-coarse, coarse, and extra-fine diamond burs, respectively. Half of the ground specimens were then heat-treated at 1200°C for 2 h to relieve the residual stresses caused by grinding and to promote reverse m→t transformation.1414. Deville S, Chevalier J, Gremillard L. Influence of surface finish and residual stresses on the ageing sensitivity of biomedical grade zirconia. Biomaterials. 2006;27(10):2186-92. doi:10.1016/j.biomaterials.2005.11.021 Specimens not subjected to abrasion or heat treatment were used as control (n = 15).

Micromorphological and surface roughness analysis

The surface roughness of each specimen was analyzed using a surface roughness tester (SJ-400, Mitutoyo, Kanagawa, Japan). Two perpendicular measurements were performed for each specimen. The arithmetic mean roughness (Ra, µm) and the maximum profile height (Rz, µm) were measured and correlated with the strength data. For the qualitative determination of the micromorphological pattern generated by grinding, the specimens were analyzed under a scanning electron microscope (SEM) (n = 2, JSM-6360, JEOL, Tokyo, Japan).

X-ray diffraction analysis of phase transformations

Phase transformations were observed by measuring the monoclinic phase fraction in each sample (n = 2) using X-ray diffraction (XRD). The data were collected with a 2θ diffractometer (Bruker AXS, D8 Advance, Karlsruhe, Germany) using Cu-Kα radiation, in a 2θ range of 20° to 65°, with step sizes of 0.5 s and 0.03. The monoclinic phase fraction was calculated using the Garvie and Nicholson’s method.1717. Garvie RC, Nicholson PS. Phase analysis in zrconia systems. J Am Ceram Soc. 1972;55(6):303-5. doi:10.1111/j.1151-2916.1972.tb11290.x

Biaxial flexural strength

The samples were subjected to a biaxial flexural strength test according to ISO 6872/2008,1616. International Organization for Standards. ISO 6872: Dental ceramic. Geneve: International Organization for Standards; 1999. in a universal testing machine (EMIC, São José dos Pinhais, Brazil). Disc-shaped specimens were positioned with the treated surface face down (tensile stress) on three support balls (Ø = 3.2 mm) placed 10 mm apart from each other in a triangular arrangement. The assembly was immersed in water, and a flat circular tungsten piston (Ø = 1.6 mm) was used to apply an increasing load (1 mm/min) until catastrophic failure. Before the test, adhesive tape (12 mm × 10 mm, 3M ESPE) was fixed on the compressive side of the discs to retain the fragments1818. Quinn G. Fractography of ceramics and glasses. Washington, DC: National Institute of Standards and Technology; 2007. and enhance the contact between piston and sample.1919. Wachtman JB Jr, Capps W, Mandel J. Biaxial flexure tests of ceramic substrates. J Mater. 1972;7:188-94. The strength σ (MPa) was calculated according to ISO 6872/2008.1616. International Organization for Standards. ISO 6872: Dental ceramic. Geneve: International Organization for Standards; 1999. After the test, the specimens were analyzed under a stereomicroscope (SteREO Discovery. V12, Carl Zeiss) to verify the location of the fracture origin.

Data analysis

One-way analysis of variance and post-hoc Tukey’s test were used to analyze the roughness and biaxial flexural strength values (α < 0.05). Roughness and strength values were evaluated as to correlation (Pearson’s correlation) (Statistix 8.0 for Windows, Analytical Software Inc, Tallahassee, USA). The latter were subjected to Weibull analysis2020. Weibull W. A statistical distribution function of wide applicability. J Appl Mech. 1951;18:293-7. to determine the characteristic strength (i.e., the strength at a 63.21% failure probability, which represents a more objective measure of strength than its average value), and the reliability of the tested material, based on the Weibull modulus to quantify the distribution of the strengths of the different samples. Weibull parameters were calculated according to DINENV843-5:1997-01.2121. Germany Institute for Standardization. DIN ENV 843-5:1997-01. Advanced technical ceramics-monolithic ceramics; mechanical tests at room temperature-part 5: statistical analysis. [Place unknown]: Germany Institute for Standardization; 1997.

Results

No correlation was found between roughness parameters and strength values (Ra, r2 = 0.0303, p = 0.7587; Rz, r2 = 0.0226, p = 0.8189). The roughness data (mean Ra and Rz values) are shown in Table 2; the grit size of diamond discs affected both the Ra (p < 0.001) and Rz parameters (p < 0.001).

Table 2
Surface roughness values of the test groups (mean and standard deviation, in µm).

Representative micrographs (Figure 1) show that surface treatment altered the micromorphological pattern after grinding, whereas heat treatment did not induce significant alterations in this pattern. The roughness analysis highlights a lower roughness for G600 (smaller granulation) than G80, G120, and GC groups, whereas heat treatment did not lead to significant alterations (Table 2).

Figure 1
Scanning electron microscopy (2,000x magnification) of surface patterns in each group. Control (Top, center); G80 (1st row, left column); G80A (1st row, right column); G120 (2nd row, left column); G120A (2nd row, right column); G600 (3rd row, left column); G600A (3rd row, right column).

X-ray diffraction data (Figure 2) show that grinding promoted a higher m-phase content than the control group, and that heat treatment induced the reverse transformation of the monoclinic phase, achieving a m-phase content similar to that of the control group.

Figure 2
Relative amount of monoclinic phase, according to X-ray diffraction analysis in each tested group: CG (4.15%), G80 (15.64%), G80A (5.97%), G120 (15.6%), G120A (5.24%), G600 (8.39%), G600A (3.27%).

Mean flexural strength and the respective standard deviations are described in Table 3. The control group presented the lowest values of flexural strength; groups without heat treatment and G120A were statistically different from the control group. The Weibull modulus (m) was not affected by abrasion or heat treatment. The characteristic strength (σc) was higher after abrasion with diamond discs, irrespective of their grit size. After heat treatment, only G80A presented strength similar to that of the control group, while the other groups sustained higher strength (Table 3).

Table 3
Mean flexural strength (FS), standard deviation (SD), characteristic strength (σc), Weibull modulus (m), and the respective confidence intervals (CI) for each group.

The fracture origin was always located on the ceramic surface and no internal defects were found. Figure 3 shows a representative image of the common fracture origin found in all tested groups.

Figure 3
Representative image (90x magnification) of the fracture origin (FO) located on the surface of the sample (Group 120A).

Discussion

The present work evaluated the effect of grinding on surface roughness and mechanical strength. The efficacy of post-grinding heat treatments to induce reverse t-m phase transformation was also investigated. Contrary to the initial hypotheses, surface roughness did not directly influence flexural strength. Based on the Ravalues, only samples ground with a 600-grit diamond disc (corresponding to extra-fine diamond burs1010. Kim JW, Covel NS, Guess PC, Rokow ED, Zhang Y. Concerns of hydrothermal degradation in CAD/CAM zirconia. J Dent Res. 2010;89(1):91-5. doi:10.1177/0022034509354193) presented lower surface roughness, regardless of the heat treatment used. In the other test groups, roughness was similar to that of the control group. Even though no clear relationship was identified between biaxial flexural strength and surface roughness, as corroborated by Sato et al.,2222. Sato H, Yamada K, Pezzotti G, Nawa M, Ban S. Mechanical Properties of Dental Zirconia Changed with Sandblasting and Heat Treatment. Dent Mater J. 2008;27(3):408-14. doi:10.4012/dmj.27.408 abrasion/polishing may be responsible for the elimination of deeper scratches produced during the manufacturing process. Polishing/abrasion eliminates a thin layer of material, also eliminating surface defects and increasing strength.2323. Aboushelib MN, Wang H. Effect of surface treatment on flexural strength of zirconia bars. J Prosthet Dent. 2010;104(2):98-104. doi:10.1016/S0022-3913(10)60100-X

Surface roughness plays a crucial role in the resistance of ceramics,2424. Jager N, Feilzer AJ, Davidson CL. The influence of surface roughness on porcelain strength. Dent Mater. 2000. 16(6):381-8. doi:10.1016/S0109-5641(00)00030-0 usually showing a significant and negative correlation with flexural strength (higher roughness with lower flexural strength)2525. Flury S, Peutzfeldt A, Lussi A. Influence of surface roughness on mechanical properties of two Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) ceramic materials. Oper Dent. 2012;37(6):617-24. doi:10.2341/11-391-L unlike the results of the present study. In addition to roughness and the amount of monoclinic phase, other factors may have influenced the flexural strength of Y-TZP discs. Abrasion/polishing may also produce deeper scratches, depending on the initial conditions of the material surface and also on the abrasion/polishing protocols. As observed in Figure 1, specimens abraded with 80- and 120-grit discs presented apparently larger and deeper scratches than those in the control group. The Y-TZP samples from the present study were polished with 1200-gritpaper before sintering. The pattern generated by the CAM milling unit was not simulated.

The influence of abrasion on the flexural strength of zirconia ceramics is uncertain. Several factors, such as roughness, plastic deformation, and the volume percentage of transformed zirconia are implicated. The last-mentioned factor depends on the rate of t→m phase transformation, on abrasion severity, and on local temperatures.22. Kosmač T, Kosmač T, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res. 2000;53(4):304-13. doi:10.1002/1097-4636(2000)53:4<304::AID-JBM4>3.0.CO;2-S,66. Kosmač T, Kosmač T, Jevnikar P, Funduk N, Marion L. The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater. 1999 Nov;15(6):426-33. doi:10.1016/S0109-5641(99)00070-6,2626. Swain MV. Limitation of maximum strength of zirconia toughened ceramics by transformation toughening increment. J Am Ceramic Soc. 1985;68:C97-9. doi:10.1111/j.1151-2916.1985.tb15305.x

After grinding, the flexural strength of zirconia discs was higher than that of the control group. This is probably due to the elimination of deeper flaws and to a more homogenous surface after grinding, and also to an increase in the fracture toughness of Y-TZP due to t→m phase transformation (Figure 2).2727. Øilo M, Gjerdet NR, Tvinnereim HM. The firing procedure influences properties of a zirconia core ceramic. Dent Mater. 2008;24(4):471-5. doi:10.1016/j.dental.2007.04.008 The surface pattern was probably standardized after grinding in G120 and G600, with or without heat treatment (Figure 1), leading to a more homogeneous stress distribution and to higher characteristic strength values when compared to the other groups (Table 3). Finer grit instruments are supposed to present, besides smaller grains, a higher number of grains and a lower distance between them, causing scratches that are greater in number and closer to each other (Figure 1), creating a more homogeneous surface than in the case of separated few scratches, probably present in G80. This last-mentioned group presented a higher characteristic strength than that of the control group, which is probably associated with the higher amount of monoclinic phase (compression layer). The characteristic strength of G80A was similar to that of the control group; heat treatment reduced the monoclinic phase content in this group (Figure 2), leading to lower flexural strength.

Heat treatment did not affect the characteristic strength in the groups in which a disc with the same grit size was used (Table 3). The enhanced strength of Y-TZP ceramic after finer abrasion with diamond devices (G120 and G600) had been reported previously,2828. Xu HKK, Jahanmir S, Ives LK. Effect of grinding on strength of tetragonal zirconia and zirconia-toughened alumina. Machin Sci Technol. 1997;1:49-66. while coarser abrasion resulted in strength reduction. Moreover, a compressive stress may still be present on the surface in these groups, even after reverse phase transformation promoted by heat treatment.

The transformation rate could also be related to the zirconia grain size: larger tetragonal grains result in lower phase stability.2929. Lee TH, Lee SH, Her SB, Chang WG, Lim BS. Effects of surface treatments on the susceptibilities of low temperature degradation by autoclaving in zirconia. J Biomed Mater Res B Appl Biomater. 2012;100(5):1334-43. doi:10.1002/jbm.b.32700 LavaTM ceramic exhibited a large grain size, as previously reported,3030. Salazar Marocho SM, Studart AR, Bottino MA, Della Bona A. Mechanical strength and subcritical crack growth under wet cyclic loading of glass-infiltrated dental ceramics. Dent Mater. 2010;26(5):483-90. doi:10.1016/j.dental.2010.01.007and thus a corresponding greater probability of phase transformation.3131. Chevalier J, Deville S, Munch E, Jullian R, Lair F. Critical effect of cubic phase on aging in 3mol% yttria-stabilized zirconia ceramics for hip replacement prosthesis. Biomaterials. 2004;25(24):5539-45. doi:10.1016/j.biomaterials.2004.01.002

The Weibull modulus of the tested groups was similar, regardless of grit size or heat treatment. This parameter describes the reliability of the material. A higher modulus indicates a small content of flaws and higher reliability.3232. Guess PC, Zhang Y, Kim JW, Rekow ED, Thompson VP. Damage and reliability of Y-TZP after cementation surface treatment. J Dent Res. 2010;89(6):592-6. doi:10.1177/0022034510363253 The procedures performed on the test samples, i.e., abrasion and heat treatment, did not produce critical defects.

The emergence of the monoclinic phase and the associated microcracking were found to be the most likely causes for the degradation of mechanical properties of zirconia-based ceramics.1212. Cattani-Lorente M, Scherrer SS, Ammann P, Jobin M, Wiskott HWA. Low temperature degradation of a Y-TZP dental ceramic. Acta Biomater. 2011;7(2):858-65. doi:10.1016/j.actbio.2010.09.020 Thus, reverse transformations from m- to t-zirconia would be desirable when stress is applied to the ceramic surface before cementation of the prosthesis. The heat treatment applied here (1200°C / 2 h)1414. Deville S, Chevalier J, Gremillard L. Influence of surface finish and residual stresses on the ageing sensitivity of biomedical grade zirconia. Biomaterials. 2006;27(10):2186-92. doi:10.1016/j.biomaterials.2005.11.021 was successful in bringing the monoclinic content after abrasion back to baseline values, as shown in Figure 2. The m→t phase transformation was also observed after heat treatment for a shorter time (1200°C / 10 min),2222. Sato H, Yamada K, Pezzotti G, Nawa M, Ban S. Mechanical Properties of Dental Zirconia Changed with Sandblasting and Heat Treatment. Dent Mater J. 2008;27(3):408-14. doi:10.4012/dmj.27.408 at a lower temperature (900°C / 1 h)22. Kosmač T, Kosmač T, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res. 2000;53(4):304-13. doi:10.1002/1097-4636(2000)53:4<304::AID-JBM4>3.0.CO;2-S, or both (930°C and 910°C / 1 min)77. Guazzato M, Quach L, Albakry M, Swain MV. Influence of surface and heat treatments on the flexural strength of Y-TZP dental ceramic. J Dent. 2005;33(1):9-18. doi:10.1016/j.jdent.2004.07.001, showing that transformation may occur after a given temperature is reached, regardless of the treatment time. The protocol applied in this study is known to relieve residual stresses without repairing cracks and/or scratches.1414. Deville S, Chevalier J, Gremillard L. Influence of surface finish and residual stresses on the ageing sensitivity of biomedical grade zirconia. Biomaterials. 2006;27(10):2186-92. doi:10.1016/j.biomaterials.2005.11.021,3333. Chevalier J, Olagnon C, Fantozzi G. Study of the residual stress field around Vickers indentations in a 3Y-TZP. J Mater Sci. 1996;31(10):2711-17. doi:10.1007/BF00687305 As shown in Table 2, surface roughness was not affected by heat treatment.

Preis et al.1111. Preis V, Schmalzbauer M, Bougeard D, Schneider-Feyrer S, Rosentritt M. Surface properties of monolithic zirconia after dental adjustment treatments and in vitro wear simulation. J Dent. 2015;43(1):133-9. doi:10.1016/j.jdent.2014.08.011investigated roughness and phase transformation (t→m) of Y-TZP after dental adjustment procedures, concluding that polishing must be carefully performed to obtain a smooth surface, with a low amount of monoclinic phase. Pereira et al.3434. Pereira GK, Amaral M, Simonetti R, Rocha GC, Cesar PF, Valandro LF. Effect of grinding with diamond-disc and –bur on the mechanical behavior of a Y-TZP ceramic. J Mech Behav Biomed Mater. 2014;37:133-40. doi:10.1016/j.jmbbm.2014.05.010 also found higher flexural strength values for Lava Frame Y-TZP when discs were abraded with finer grit instruments. The polishing/abrasion performed in this study (diamond discs attached to a polishing machine) was controlled for direction and applied load. This may lead to different damage when compared to clinical adjustments, which are not under any kind of load or direction control. For this reason, the protocol used in this study was probably less deleterious to the material properties than the ones used by clinicians.

Clinically, the adjustment should be performed with fine diamond burs to avoid the decrease of mechanical properties and, consequently, the premature failure of all-ceramic restorations. Further investigations are needed to evaluate the effects of abrasion and heat treatment, as well as of the amount of monoclinic phase, on the long-term mechanical behavior of zirconia.

Conclusion

When the clinical fine adjustment of zirconia restorations is necessary, it should be performed with finer grit diamond burs (120 and 600 grit sizes, corresponding to coarse and extra-fine diamond burs), thus not affecting the efficacy of the restorations. Heat treatment can be applied after the adjustment when absence of the monoclinic phase is desirable. The heat treatment protocol (1200°C / 2 h) employed in this work proved to be effective.

Acknowledgments

The authors thank the Research Foundation of the State of São Paulo (Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP) for the financial support (FAPESP - 2011/16716-0).

References

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Publication Dates

  • Publication in this collection
    2016

History

  • Received
    25 Jan 2015
  • Accepted
    19 June 2015
  • Reviewed
    27 Aug 2015
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